The present invention is directed, in general, to a micro-electro-mechanical device and, more specifically, to a micro-electro-mechanical device having improved torsional members and a method of manufacture therefore.
Optical communication systems typically include a variety of optical devices, for example, light sources, photo-detectors, switches, cross-connects, attenuators, mirrors, amplifiers, and filters. These optical devices transmit optical signals within the optical communications systems. Some optical devices are coupled to electro-mechanical structures, such as thermal actuators, forming an electro-mechanical optical device. The term electro-mechanical structure, as used herein, refers to a structure that moves mechanically under the control of an electrical signal.
Some electro-mechanical structures move the optical devices from a predetermined first position to a predetermined second position. Cowan, William D., et al., xe2x80x9cVertical Thermal Actuators for Micro-Opto-Electro-Mechanical Systems,xe2x80x9d SPIE, Vol. 3226, pp. 137-146 (1997), describes one such electro-mechanical structure useful for moving optical devices in such a manner.
These micro-electro-mechanical systems (MEMS) optical devices often employ a periodic array of micro-machined mirrors, each mirror being individually movable in response to an electrical signal. For example, the mirrors can each be cantilevered and moved by an electrostatic, piezoelectric, magnetic, or thermal actuation. See [articles by] Lin, L. Y., et al., 10 IEEE Photonics Technology Lett. 525 (1998); Miller R. A., et al., 36 Optical Engineering 1399 (1997); Judy, J. W. et al., A53 Sensors and Actuators 392 (1996), which are incorporated herein by reference.
FIGS. 1A and B illustrate a prior art MEMS optical device 100 and its application. The device 100 comprises a mirror 110 movably coupled to a surrounding silicon frame 130 via torsional members 150, a gimbal 120 and torsional members 180. Typically, the mirror 110 includes a light-reflecting mirror surface 160 coated over a polysilicon membrane 170, which is typically of circular shape. The two torsional members 180 on opposite sides of the mirror 110 connect the mirror 110 to the gimbal 120, defining the mirror""s primary axis of rotation. The gimbal 120, in turn, is coupled to the surrounding silicon 130 frame via a second pair of torsional members 150 (FIG. 1B) defining a secondary axis of rotation orthogonal to that of the primary axis of rotation.
These components are fabricated on a substrate (195) by known micromachining processes such as multilayer deposition by evaporation, sputtering, electrochemical deposition, or chemical vapor deposition and subsequent selective etching. Some of the polysilicon layers used in this process are separated spatially by phosphorous-doped silica glass. This glass is used as a sacrificial layer that is removed during an etching process sometimes referred to as the release process.
After etching, the components, are raised above the Substrate 195 by lift arms 140 that are composed of two dissimilar metal layers deposited over a polysilicon base. Alternatively, the etching process, may produce vertical support structures. In either case, the mirror 110 or gimbal 120 may be rotated out of plane of the support structure by an electrostatic force applied between the mirror 110 and electrode 190, and the rotation may be controlled by the restoring force of the torsional members 150, 180. Thus, using the typical MEMS mirror, the light beam of the optical signal can be transmitted to other devices in the optical system.
However, one problem associated with the fabrication of such devices relates to the corrosion of the polysilicon containing components and in particular, the torsional members 150, 180, during conventional etching (i.e., release) processes. In optical applications, various metals need to be applied to meet the optical and mechanical properties of the device. The use of multiple metals on silicon and the subsequent immersion of such a structure in a highly conductive and corrosive etching solution (in this case hydrofluoric acid; HF) can result in multiple electrochemical reactions. These reactions can accelerate the etching of a particular material in the structure.
The controlled release of silicon micromachines by HF has been well-documented. See D. J. Monk, Controlled Structure Release for Silicon Micromachining, Ph.D. Dissertation Thesis, University of California at Berkeley, passim and references sited therein (1993), all of which are incorporated herein by reference. The general chemical reaction that occurs is as follows:
SiO2(s)+6HF(aq)xe2x86x922H2O(l)+H2SiF6(aq)
In this case, HF is the reactive species. It is also known that solutions of HF also contain H3O+, Fxe2x88x92 and HF2xe2x88x92, with the concentrations of each species dependent on the bulk concentration of HF. Fxe2x88x92 and HF2xe2x88x92 are known to react with silicon under certain conditions, and the reaction of Fxe2x88x92 with silicon liberates electrons. When noble metals such as gold are coated on the silicon, an electrochemical cell is created wherein the fluorine attacks silicon liberating electrons which migrate to the gold surface and reduce protons to produce hydrogen gas. This structure then accelerates the attack of fluorine on silicon. It is believed that phosphorus doping accelerates the attack of fluorine on silicon by weakening the silicon-silicon bonds. See Torcheax, L et al., 142(6) J. Electrochemical. Soc. 2037 (1995); Chung, B. C. et al., 144(2) J. Electrochemical. Soc. 652 (1997); Bertagna, V. et al., 146(1) J. Electrochemical. Soc. 83 (1999), all incorporated herein by reference.
In the example described above, corrosion has been found to affect the properties of the MEMS mirrors due to changes in the torsional member""s mechanical properties. Such changes may cause the finished device to fail to meet desired performance specifications. Even where such finished devices are within design tolerances, corrosion and degradation may cause the response of each mirror in the optical system to vary largely when the actuating means is applied. Clearly, such characteristics are not desirable.
Accordingly, what is needed in the art is a MEMs device that does not suffer from such degradation during the method of manufacture therefor.
To address the above-discussed deficiencies of the prior art, the present invention provides a micro-electro-mechanical (MEMS) device that includes a moveable element, such as a mirror, supported by a moveable element support structure and torsional members connected to the moveable element that are substantially free of corrosion. These improved torsional members allow the moveable element to be moved relative to the support structure in a more efficient and accurate manner.
The present invention also provides a method of fabricating a micro-electro-mechanical device that includes forming a plurality of layers on a supporting substrate wherein at least a portion of one layer is a sacrificial layer, defining at least one torsional member in at least one layer of the layers that is not the sacrificial layer. A moveable element is defined in at least one of the layers that is not the sacrificial layer, wherein said moveable element is attached to the torsional member. The sacrificial layer is then removed to permit the movable element to move relative to the substrate. The structure is formed from a combination of dissimilar materials (e.g. the moveable element is a silicon or polycrystalline material on which a metal is formed). In the context of the present invention, dissimilar materials are materials with different reduction-oxidation (RedOx) potentials that create an electrochemical cell in the presence of the etchant for removing the sacrificial layer. In the embodiment of the present invention wherein the moveable element is a gold plated mirror and the torsional member is polycrystalline silicon, the corrosion of the polycrystalline silicon member by the etchant is enhanced by the presence of the gold in the structure. According to the present invention, the sacrificial layer is removed using an etchant mixture including an anion suppressant, wherein the anion Suppressant is selected to inhibit corrosion of the torsional member by the etchant solution.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.